1. Field of the Invention
The present invention relates to the irradiation, with charged particles, of a semiconductor substrate on which a semiconductor device is formed, to control the electrical characteristics of the semiconductor device.
2. Description of the Related Art
Power devices such as power metal-oxide-semiconductor field-effect transistors (MOSFETs) and insulated gate bipolar transistors (IGBTs) are typically formed on a semiconductor wafer. It is known that the response characteristics of these devices can be improved by irradiating the wafer with a high-energy beam of charged particles such as ions or electrons. Irradiation with charged particles produces lattice defects in the wafer. By functioning as carrier recombination centers, the lattice defects promote the recombination of electrons and holes, thereby shortening the mean carrier lifetime. Charged particle irradiation therefore enables control of the turn-off time of a power device, and improves other switching characteristics.
During irradiation by the charged particle beam, wafers are placed in a carrier device such as the tray disclosed by Urushima et al. in Japanese Patent Application Publication No. 2003-168731. Even when wafers are placed in trays of identical structure and irradiated with charged particle beams under identical conditions, however, wafer-to-wafer variations are found to occur in electrical characteristics such as the gate threshold voltage of power devices formed on the wafers. Such variations lead to lower production yields and higher device failure rates in the field.